The present invention relates to a method for the preparation of a sintered body of a rare earth oxide. More particularly, the invention relates to a method for the preparation of a sintered body of a rare earth oxide having a large sintering density and a small average crystallite diameter. The invention also relates to a method for the preparation of an oxide mixture mainly consisting of a rare earth oxide with a non-rare earth adjuvant oxide, which crystallographically consists of a single phase.
As is well known, sintered bodies of a rare earth oxide generally have an outstandingly high corrosion resistance against halogen gases or halogen-containing gases and melt of a metal or alloy. By virtue of this unique property, applications in various fields can be expected for the articles of a sintered rare earth oxide body. Besides, sintered bodies of a rare earth oxide are each a potential material in the applications as a dielectric material, magnetic material, optical functional material and so on. For example, yttrium aluminum garnet, referred to as YAG hereinafter, having a chemical composition of Y3Al5O12 as a composite oxide of yttrium and aluminum belongs crystallographically to the cubic system having isotropy and a high-density sintered body of YAG, which is nothing other than a polycrystalline body, may exhibit high transmissivity to visible light close to that of a single crystal or glassy body of YAG. Further, sintered bodies of a composite oxide consisting of a rare earth oxide, such as oxides of yttrium, dysprosium and terbium, and iron oxide have an application as a material of magnetooptical devices.
It is important in most applications of a sintered body of rare earth oxides in order to exhibit the inherently high performance that the sintered body has a sintering density, i.e. the actual density of the sintered body relative to the true density of the oxide, as close to the true density of the oxide as possible and that the sintered body consists of a crystallographically single phase. These desirable characteristics of a sintered body of a rare earth oxide largely depend on the physical properties of the starting oxide particles and it is generally a very difficult matter to obtain a sintered body of a rare earth oxide even by undertaking improvements and optimization of the process conditions for the preparation of a sintered body.
The present invention accordingly has an object, in view of the above described problems and difficulties in the prior art methods for the preparation of a sintered body of a rare earth oxide or an oxide mixture mainly composed of a rare earth oxide or a rare earth oxide-based oxide mixture, to provide a novel and reliable method for the preparation of a sintered body of a rare earth oxide or a rare earth oxide-based oxide mixture having a large sintering density and consisting of a crystallographically single phase with a small average crystal-lite diameter.
Thus, the method of the present invention for the preparation of a sintered body of a rare earth oxide comprises the steps of:
(a) molding a powder of a rare earth oxide, of which the D50 value of the particle diameter distribution does not exceed 2.0 xcexcm, the D90 value of the particle diameter distribution does not exceed 3.0 xcexcm and the specific surface area is in the range from 5 to 20 m2/g, into a powder compact; and
(b) subjecting the powder compact to a heat treatment for sintering at a temperature of 1000xc2x0 C. or higher, in which the rate of temperature elevation does not exceed 500xc2x0 C. per hour and the rate of temperature decrease does not exceed 600xc2x0 C. per hour.
It is preferable in the above defined inventive method that the Dxe2x80x250 value of the pore diameter distribution of the rare earth oxide particles does not exceed 20 nm.
It is further optional that the starting rare earth oxide powder defined above is admixed with an adjuvant oxide of an element selected from the group consisting of magnesium, aluminum, silicon, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, gallium, germanium, zirconium, niobium, molybdenum, indium, tin, hafnium, tantalum and tungsten in a limited proportion so as to give a sintered body of a rare earth-based composite oxide. When a sintered body is prepared from an oxide mixture of a rare earth oxide and one or more of these adjuvant oxides, it is desirable that the amount of the rare earth oxide is at least 40% by weight based on the total amount of the oxide mixture.